This invention relates to amplifying arrangements employing klystrons or like devices and more particularly, but not exclusively, to such arrangements comprising television transmitters.
In a television transmitter typically a klystron amplifies a radio frequency carrier, which has been modulated with either a video signal, or with video and sound signals simultaneously, the latter mode of operation being known as a "multiplexed" mode.
A "klystron" is a device for amplifying high frequency signals. A typical klystron comprises an electron gun section and an interaction region. The electron gun section includes a cathode at which electrons are emitted in a beam and a beam controlling electrode which controls the magnitude of the electron beam current. The interaction region comprises a cavity at which the high frequency signal to be amplified velocity modulates the electrons of the beam, and typically comprises several cavities, the amplified signal being taken from the final one.
In a previous high frequency amplifying arrangement comprising a television transmitter, the modulated radio frequency carrier is applied to an input cavity of the klystron. Since a klystron is not a linear amplifying device, the signal modulating the radio frequency carrier must be distorted before the modulated carrier is coupled into the klystron to correct for the non-linearity and obtain satisfactory transmitter performance. However in such an arrangement the klystron electron beam current is at a constant high level throughout operation since it must be sufficient to enable the required output power for transmission of the synchronising pulses to be obtained. This results in low operating efficiencies during the picture period of the television signal, the picture period being approximately 93 percent of the total operating period. The klystron radio frequency output power required during the picture period is much lower than that required during the synchronising pulse period. Thus the operating efficiency of the klystron may be improved by pulsing the beam current so that it is high during the synchronising pulse period and lower during the picture period, giving a reduction in the average klystron beam power.